1. Technical Field
This disclosure relates to a method and apparatus for processing a received multipath signal, in particular for determining the earliest significant signal path. The method and apparatus have application in, for example but not exclusively, the Universal Mobile Telecommunication System (UMTS) and in particular the High Speed Downlink Packet Access (HSDPA) service of UMTS.
2. Description of the Related Art
The HSDPA scheme has been introduced in Release-5 of the UMTS standard to provide a high data rate service. For previous releases of the UMTS standard, such as Release-99, a suitable receiver architecture is considered to be the well known Rake receiver. For HSDPA, because of the high data rate and due to the dispersive nature of the channel, a Rake receiver is considered less suitable and a receiver employing an equalizer is considered preferable. The equalizer needs information about the earliest significant signal path, referred to in the present specification and claims as the first significant path (FSP), of a received multipath signal. This is in contrast to a Rake receiver which employs the timing of all the paths. The present disclosure addresses the provision of information about the FSP.
In the UMTS Frequency Division Duplex (FDD) downlink and in other similar systems, although the signals transmitted from a base station (BS) to different mobile stations are orthogonal, this orthogonality no longer exists at the mobile station (MS) receiver due to the multipath effect of the propagation channel between the transmitter and the receiver. This loss of orthogonality causes inter-code interference, inter-chip interference and inter-symbol interference in the symbol estimates. Receivers that are within the optimal or close-to-optimal category, such as multiuser detectors (MUDs) and interference cancellers (ICs), most of the time require knowledge about the signal and the channel parameters of all active users so as to mitigate the multipath effect and detect the desired data stream reliably. However the possibility of implementing MUDs or ICs in mobile stations is limited due to their high complexity and due to the fact that the transmission parameters of all the users are usually not known. A very practical and highly utilized suboptimal solution is the conventional Rake receiver that performs a matched filter operation on the code of the desired user, such that multi-user interference is considered as an additional white noise. However, when small spreading factors are used to achieve high data rates, as for example for HSDPA, the performance of the Rake receiver decreases due to the fact that the multipath interference becomes significant and the correlation characteristics of the spreading sequences are destroyed. For these reasons, the use of equalizers is considered for the HSDPA scheme in order to restore the orthogonality between the users and limit the interference, allowing a higher data rate to be achieved. For HSPDA a suitable equalizer is a variant of the Normalized Least Means Squares (NLMS) equalizer, implemented as an adaptive finite impulse response (FIR) filter and which requires the FSP to be placed near the center of the equalizer window. If HSPDA is implemented on a co-processor, the location of the FSP may be signalled to the HSDPA co-processor by the host on which it resides, and the location may be periodically updated.